In the fields of optics in general including spectroscopy and optical communications, a light source offering a broadband tunability has been eagerly desired. Although the term “broadband” is generic and defined only vaguely, it here points to a band from 800 to 1800 nm that is of importance in the fields of spectroscopy and optical communications. In many applications, an incandescent lamp or the like combined with a spectrometer has been the optimal choice as a broadly tunable light source; the intensity of light remains no more than a few tens of μW per 1 nm, and even worse, convergence of light is not as good as that of coherent light like laser radiation. This is solely attributable to incoherence of light generated in an incandescent lamp.
Driven by the same desire, several lasers as a coherent light source having a fair tunability have been developed. In fact, some of them have been put in practical use in the many and expanding fields of application.